fix previous commit for xml

Author: huard

owslib/feature/postrequest.py

@@ -175,8 +175,10 @@ def set_filter(self, filter):
         """
         if isinstance(filter, str):
             f = etree.fromstring(filter)
-            # If this raises an error, it means the filter string does not contain an actual Filter node.
-            sub_elem = f.xpath("//FES:Filter", namespaces={"FES": FES_NAMESPACE})[0]
+            if f.tag == util.nspath("Filter", FES_NAMESPACE):
+                sub_elem = f
+            else:
+                sub_elem = f.find(util.nspath("Filter", FES_NAMESPACE))
 
         else:
             sub_elem = filter